singleton variable

A variable which is only referred to once in a
piece of code, probably because of a programming mistake. To
be useful, a variable must be set and read from, in that
order. If it is only referred to once then it cannot be both
set and read.

There are various exceptions. C-like assignment
operators, e.g. "x += y", read and set x and return its new
value (they are abbreviations for "x = x+y", etc). A
functionargument may be passed only for the sake of
uniformity or to support future enhancements. A good
compiler or a syntax checker like lint should report
singleton variables but also allow specific instances to be
marked as deliberate by the programmer.